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MAC.RO(Maccaferri Rockfall Protection System)

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1) Type of surface instability

2) Mitigating effects using a rock fall barrier system

3) Mitigating effects using a netting system

4) Considerations about the performance of a drapery system

ProblemTo analyze how instability is originated and methodologies for a proper use of drapery and rock fall barrier systems.

Factors affecting the performance of drapery and rock fall barrier systems

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SURFACE INSTABILITY OF ROCK SLOPES

Relates to the external surfacewithout affecting the overall slopestability

Concerns only the loose portion of the rock surface

In few cases they can be described bysimple kinetic mechanisms

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SURFACE INSTABILITY OF SURFACE INSTABILITY OF

ROCK ROCK SLOPESSLOPES

SURFACE INSTABILITY OF

SOIL SLOPES

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TYPES OF POSSIBLE INSTABILITY

Falling of small blocksFalling of large blocks

Instability of the whole slope

A B

Collapse and deep instability

C D

Surface instability: relates to external rock surfaces without affecting the overall stability of the slope

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PASSIVE PROTECTION Mitigation of instability effects

ACTIVE PROTECTION Instability prevention

PROTECTING FROM SURFACE INSTABILITY

CORTICAL STRENGTHENING

LOCAL CONSOLIDATION BY NAILING-ANCHORING

SIMPLE DRAPERY SYSTEMS

TRENCHES, EMBANKMENTS, BARRIERS

SOIL NAILING

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MORPHOLOGY OF UNSTABLE SLOPES

Obtain actual site measurements C) Vertical slope

A) Regular slope B) Inclined slope with some vertical steps

D) Intermediate / complex situations

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ROCKFALL

PARAMETERS AFFECTING THE DESIGN OF BARRIERS AND EMBANKMENTS :

FALLING ENERGY FALLING

VELOCITY

HEIGHT OF IMPACT

BEST LOCATIONALONG THE SLOPE

OPERATIVE PROBLEMS DURING THE

INSTALLATION

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FALLING VELOCITY

A=Broken barrier

A = Sacrificial Barrier B = Effective barrier

Higher velocities (i.e. 35-40 m/s) are assumed to make one barrier collapse under the impact.

In such case multiple barrier lines shall be used: the first sacrificial line will break dissipating the block, the second barrier will stop it.

Barriers are certified for velocities up to 30 m/s.

VELOCITY VS HEIGHT OF FREE FALL

10

15

20

25

30

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0 20 40 60 80HEIGHT OF FREE FALL [m]

VELO

CIT

YO

FFA

LL[m

/s]

range ofvelocity on slope

limit

of fr

ee fa

ll fo

r bar

rier

high velocityafterwardsimpact and explosion of boulders

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FALLING ENERGY

Energy of a falling rock:

E = Eϖ + Ek

Ek = translational energy = ½ M v2

M = block massv = translational velocity

E ω = rotational energy = ½ I ω2

I = inertia momentω = angular rotational velocity

The most effective energy is the translational one which is normally 80% (or higher) than the total kinetic energy.The rotational energy is around 10-20% of the total energy and is related to the shape of the

block.

0

500

1000

1500

2000

2500

3000

5 15 25 35 45 55

HEIGHT OF FREE FALL [m]

ENER

GY

[kJ]

1.0 m3

1.5 m3

2.0 m3

3.0 m3

'

TRANSLATIONAL ENERGY VS HEIGHT OF FREE FALL

05 15 25 35 45

NR

G[

J]

1.0 m3

1.5 m3

2.0 m3

3.0 m3

'

range ofused

barriers

range ofused

barriers

frequ

ent

no fr

eque

nt

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FALLING ENERGY

h

E ≅ 1/ 3 m g h

E = potential energy

m = mass

g = gravity 9.81 m/s2

h = falling height

VELOCITY VS VOLUME OF A FREE FALL BOULDERWITH CONSTANT ENERGY LEVEL

-

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

13.0

14.0

15.0

10 15 20 25 30VELOCITY (m/s)

VO

LUM

E(m

3)-W

EIG

HT

26.5

kN/m

3

E=250

E=1000

E=2000

range ofused

barriers

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HEIGHT OF IMPACTTopographic section with the rock tracks

STEP

STEP

FLY

FLY

The barriers must be higher than the path of falling boulders.

We must take into account:

A) A statistical approach cannot forecast 100% of the events

B) Simulation gives the trajectory without considering the actual boulder dimensions.

C) There is ratio between the height of a rockfall barrier and its energy dissipating capacity

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Topographic sectionwith the rock tracks

Slopes with moderate gradient: Structure: Embankment(double sided Reinf. Soil. Structures to limit space occupancy)

Slopes with steep gradient: Structure: Flexible barrier.

Slopes with very steep gradient: Structure: Flexible barriers or Rockfall canapee (shelter), in case of energy dissipation requirements higher than 3000 kJ

LOCATION AND TYPE OF STRUCTURE ALONG THE SLOPE

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AB

C

D

The most favorable morphology should be chosen:

Barriers are less effective when built in ditches or at the bottom of vertical rock slopes.

high: boulders have low velocity; they roll and make a series of low jumps. The assembly of the barrier is easy because close to the road

D

high: boulders have low velocity; they roll and make a series of low jumpsC

low: boulders can jump over the barrierB

low: boulders with high velocity can pierce the barrier. Boulders can jump over the barrier

A

QUALITYLOCATION

LOCATION ALONG THE SLOPE

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For a rockfall barrier location, the following needs to be verified:

Minimum distance between barrier and road (or building) to be protected

Presence of walls, slopes

Irregularities of the slope profile

Rockfall barriers shall be installed at more than 10 m from road or infrastructures.Good locationD

Barrier foundation in proximity of a retaining structure. Not admittedC

The boulder can skip the barrier. Not admittedB

Too close to the road. Not admittedA

QUALITYPOSITION

SOIL

WALL

WALL

BOULDER

SOIL

ROAD

D C B A

LOCATION ALONG THE SLOPE

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INSTALLATION ISSUES

Is it possible to remove unstable blocks by scaling?

Is it possible to cut treesbefore installation of the mesh or rockfall barriers?

Are there access roads?Where are they in relation to the slope to be protected?What are the installation requirements?

Road

Are there any restrictions regarding possible use or need to use a helicopter ?

(presence of electrical lines or cables etc.)

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POST INSTALLATION ISSUES

Maintenance ensures efficective protection.

The customer must be informed about maintenance requirements.

1. Periodical visual inspections to check barriers

2. Periodical removal of the debris at the back of barriers or embankments

3. Repair of the active-passive protections (and check the state of the anchoring system )

4. Check the dissipation system especially when under heavy load from high level of snow

5. Check barrier after fire events

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Advantages and limitations of drapery systems

Low power equipment normally can be used

Uncertainty of the geomechanical model

Low durability and high maintenance costs

It only affects rock surface instability

Reasonably low costs

No special site equipment required

Easy to install

Low visual impactEasy to design

Can solve most common types of instability

ADVANTAGES LIMITATIONS

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Netting performance

Wire mesh tensile and punching resistance

Steel tensile strength

Stiffness (elongation)

Different performance in the cross directions (anisotropy)

Resistance to chemical degradation

Difficulties in the installation process

KEY FACTORS AFFECTING INSTALLATION

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Deformation of the mesh

The load direction results by the movement of small blocks

The wire net will deform because of:

1) Uneven pressure on the mesh due to the slope irregularities

2) Untightened anchors allowing mesh movement around it

3) Strain of the mesh

The steel resistance will start to be effective when all deformations with small loads have occurred.

The higher the stiffness the more effective the drapery system iThe higher the stiffness the more effective the drapery system iss

KEY FACTORS AFFECTING INSTALLATION

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Most used types of netting.

Cable mesh panels

Simple twist wire

mesh

Double twist wire

mesh

Steelgrid

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LOAD TRANSFER

Load is transferred to the closest anchor points.

S.T. WIRE MESH PANELS CABLE MESH PANELS

Load is transferred to the anchor points alongdiagonal directions

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BOUNDARY CONDITIONS

Confininglines

anchors

Cable mesh panels: load is transferred along the “diagonal directions”. The border cable doesn’t improve the performance

Wire mesh rolls: connection along the longitudinal boundaries is important

⇓

Steelgrid provides additional restraint through the longitudinal cables

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DEFORMABILITY OF MESHHexagonal wire mesh an cable panel

High strength is developed at low strain rates, enabling more uniform stress across the netting plane

Best to provide high surface holding capacity.

Simple twist wire mesh

High strength is developed at higher strains, when fractured rock is partially (or already) dislodged.

Best to build fences.

-

+de

form

a bil i

ty

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Around anchorpoints the stress on the wire mesh maybe fairly high.

Wire mesh strength on the anchor points

Excessive stiffnesson the single wiremay result in localwire failures!

The mild steel wireused in the doubletwisted mesh willstrain withoutrupturing.

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Ductility vs slope morphology Wire mesh drapery systems require to conform to the slope contours to perform effectively.

Double twisted mesh may be easily stretched sideways minimizing overlapping areas along the joint lines, hence minimizing the quantity of rolls improving the effectiveness of the adhesion at the same time.

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Manual drilling: anchor depth up to 3-4 m (typical for reinforced drapery systems)

Light Drilling

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Direction of the perforation downwards: typical of 10-20m anchor depths, up to30m depth for weak rock soils. Used toconsolidate slopes (sometime in reinforced drapery systems as well)

Deep drilling

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Anchor performanceSteel tensile strength (yielding) and shear resistance

Typical steel bar characteristics (threading, hollow bars, available attachments…)

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Concrete grouting of anchors

Practical solutions for operations on rock face

Adhesion of grouting to the rock

Resistance to chemical / physical degradation

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??

Correct installation practicesLack of reference standards or regulations may lead contractors to perform incorrect installations.

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trench

Simple drapery system with longitudinal

cables

Simple drapery systems are provided with top anchors.

Top anchors don’t have structural function.

Wire mesh mainly holds the small size rocks (up to 0.5m diameter for the double twisted mesh ).

A trench at the bottom of the slope is always recommended.

Simple drapery system with

trench

Simple drapery system with vertical

cablesVertical cables woven in the wire mesh (SteelGRID) enable to transfer more load to the upper anchors improving the drapery overall effectiveness.

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It is not relevant to consider the dynamic action of blocks falling between the mesh and the slope:

it is considered only in particular applications or it means that the design was not accurate.

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Cortical strengthening

Cortical strengthening systems are provided with a regular anchor pattern.

Anchors are placed to improve the stability of the rock surface.

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Background:Underground a few cms of sprayed shotcrete is sufficient to stabilize an excavation and prevent block detachment.

The shotcrete is enough to prevent the mutual movement between the blocks and therefore increase the rock mass resistance (Hudson & Harrison, 1997).

Conceptually the mechanism to be considered to consolidate a superficial instability is the same: to prevent the movement between the blocks.

For slope cortical strengthening the function of shotcrete is performed by the anchors. The function of the mesh is to develop a local holding effect for the small rocks and transfer the holding effect across the anchor points for the larger blocks

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Thank youfor your kind attention